Ok... first off, many here ARE indeed diehard Pontiac only fans... and many are All original or nothing kind of guy's. But you will find that anyplace with most any brand...
As you said, you have the 408 and a TH400 already, and if you are saving a First gen from another fait, then by all means save a bird! Do what ever it takes!
If you take a good look, the Pontiac Firebird was a higher end product than the Camaro was, or at least that's how it was from the beginning. And since a Firebird is harder to come by than a Camaro is... Go For it, be different from all the Big Buck Camaro guy's!
And I'm pretty sure the cross members are about the same. You'll need a mount kit obviously and a few extras... but all in all it should be pretty straight forward! Just make sure you get the tranny cross member for the TH400.
And the reason there is not a lot of traffic here as apposed to Nastyz28.com or some of the others is this site has an excellent mailing list forum that everyone uses... )except Bjorn... he's banned for talking too much! Lol...) (ISP problems) But that is where all the posts, Q&A really are! And it comes directly to you!
Anyway, Rip-It-UP, go for it, save a 1st Gen Bird no matter what it takes. Yes you'll get some flack from purists... but isnt it all worth it? With a big block your looking to be different anyway?